The most popular conventional fastener of a post-installation type is constituted by a sleeve having an expansion portion and a plug (expanding member) having a taper portion for expanding the expansion portion by being inserted into the sleeve, and is basically arranged to fix the sleeve as an anchor to a concrete structure by hammering the plug or sleeve upon installing the fastener into a prepared hole and by expanding the expansion portion by a relative displacement therebetween.
In a structure of such a conventional fastener, since the sleeve and the plug are merely press-fitted under a non-expanded condition of the expansion portion, they tend to be separated. Specifically, when it is installed upwardly, there is a possibility that the operation ability during the installation is degraded by the detaching of the sleeve. Further, since there is no means for restricting a relative position between the expansion portion and the plug even under a condition that the expansion portion is expanded, slipping out of the plug from the sleeve or detaching of the sleeve from the prepared hole tends to be generated by the generation of the loosening between the sleeve and the plug in the case that it receives earthquake or mechanical vibration energy. This degrades the anchor effect and is not suitable.
On the other hand, as a method for working an undercut hole acting as a prepared hole prior to the installation of the post-installation anchor of an undercut type, there are a one-step method and a two-step method.
The former one-step method is, as shown in FIG. 65, a method for working an undercut hole 224 by first working a straight prepared hole 222 using a dedicated drill bit 220 installing a stopper 221, and by enlarging a hole bottom portion of the straight prepared hole 222 into a taper shape in a manner of executing a planetary operation for swinging and rotating the drill bit 220 at the stepper 221 as a fulcrum by several rotations after a depth of the prepared hole 222 reaches a predetermined depth and the stopper 221 is contacted with a concrete surface 223.
The latter two-step method is, as shown in FIG. 66, a method for enlarging a portion near a hole bottom of a straight prepared hole 225 into a taper shape by first working the straight prepared hole 225 by means of a known working method, by seating a special drill bit 226 for diameter-enlarging operation, and by enlarging a tip end of a cutter 227 of the drill bit 226 utilizing a reaction force from the hole bottom against the drill pressing while putting the drill bit 226 at the hole bottom.
However, although the former method can drill an undercut hole by one step, it requires a special operation of executing a planetary operation of the drill bit 220 integrally with an electric drill main body. Therefore, this requires a skill in this drilling operation. If a builder is not skilled in this operation, there will be obliged to degrade an accuracy of a hole working and to generate a breakage of the drill bit. Particularly, there is a drawback that a dispersion as to hole working accuracy is large due to individual differences.
Further, the latter method requires two steps as called, and requires to change the drill bit 226 or to change the handling of an electric drill. Therefore, there is a drawback that the steps for working the hole and the operation time are dully elongated.